According to the technique classically used in the oleaginous seed industry, small seeds are not decorticated and the oil in the kernel is extracted by pressing or by a solvent. The coat of the kernel is cumbersome and often unusable, and has an unfavorable effect on the extraction yield and grade of the end products and byproducts.
In the mustard industry, the seeds are crushed in mills with the verjuice in which the pulverized kernels are suspended. After crushing, the product obtained is filtered. The fine fraction constituting the mustard is separated while the coarse fraction containing the coats soaked with verjuice, and a not negligible part of the potentially usable raw material, is rejected as waste.
This process has several disadvantages. The discharge of waste carrying away a relatively substantial proportion of the oil causes a drop in yield by comparison with the volume of the seed's kernel. In addition, the screens used for filtering, although they are made of stainless and hence expensive steel, have a limited service life because they are attacked by the acid in the verjuice. In addition the waste product, containing the coat debris, causes blockages in the sewage system.
French Pat. No. 2,032,032 proposed a process consisting of freezing the seeds to solidify their fat content and provide the seed decortication operation at low temperatures such that the fats are solid during the entire operation. According to this process the mustard seeds are cooled to a temperature of about -25.degree. to -35.degree. C before decortication, the decortication chamber itself being held at a temperature of about -20.degree. to -30.degree. C.
In another technological area, it is known from D. Come and T. Tissaoui, "Revue generale du froid," No. 3, March 1973, that the mechanical effect of intense cold on the integuments of hard seeds, particularly leguminous seeds, has beneficial consequences since it permits better penetration of water when the seeds are allowed to germinate. D. Come considers that the impermeability of the integuments to water resides in the most superficial layer. It is thus likely that it is only needed to cool the surface down to a temperature of about -40.degree. to -50.degree. C to cause microcracking; controlled embrittlement of the coat must be rapid.